This invention relates to a brake assembly for a wheel.
A conventional disc brake assembly for a wheel of a vehicle comprises a cast iron or steel disc rotor (ventilated or solid) which is either joined or cast integrally with the wheel carrying hub and bearings to facilitate fitting to the axle stub, and when fitted the disc rotates with the wheel assembly. A brake caliper is then positioned such that it straddles each side of the disc rotor. This brake caliper assembly consists of a main body, usually either cast iron or cast alloy, which holds disc brake friction material pads in a position in relation to the disc such that the pads can be applied to each side of the disc rotor face. The brake caliper is manufactured such that it has the provision for one or more pistons which are slidably received into the caliper body. The caliper is hydraulically actuated from a master cylinder assembly which displaces brake fluid from a master cylinder through a brake line to the caliper. The brake fluid is introduced into the caliper under high pressure and results in displacement of the pistons in the caliper body out towards the disc face. The pistons act upon the friction material pads which result in the pads being forced against the disc. In practice these pistons have always been manufactured from steel or aluminium and the final finishing of them effected by either chrome plating or anodising.
A problem with steel hydraulic pistons is that they are usually very conductive to heat transfer. Thus upon continued braking of a wheel, the friction pads generate extreme heat which is then conducted by the hydraulic pistons and the caliper body to the brake fluid add to the seals. In order to minimize the possibility of such heat transferral causing the hydraulic brake fluid to vapourize, it has been necessary to design a brake fluid with increasingly higher boiling points. The currently acceptable disc brake fluid carries a nominated boiling point of approximately 270.degree. C. This boiling point is only some 30.degree. C. below the degradation point of the synthetic rubber seals which hydraulically seal the piston from the caliper body. If the brake fluid is contaminated with small amounts of water the boiling point can decrease to a about 140.degree. C. It is a property of most brake fluids that they are hygroscropic and thus readily absorb moisture resulting in a significant decrease in the boiling point of the brake fluid. This can result in potentially dangerous brake failure known as "vapour lock" which occurs when the heat transferred from the disc pads to the hydraulic brake fluid results in the formation of small pockets of vapour.
Chromium plated steel pistons are the most common is disc brake systems but aluminium and titanium may be used in specialised cases. The aluminium and titanium pistons have the advantage of light weight and less corrosion compared to steel, but have a much higher thermal conductivity.
A further disadvantage with conventional pistons is that corrosion or pitting can occur on the surface of the piston or caliper due to environmental conditions or contaminated brake fluid. Such corrosion results in roughening of the surface of the piston which subsequently results in an accelerated degradation of the hydraulic seals by abrasion.
Furthermore engineers make a considerable effort to reduce the unsprung weight of automotive suspensions. With light weight pistons, the inertia of the braking system is reduced, leading to quicker reaction, as well as lower unsprung weight. As mentioned above, aluminium or titanium may be used in the pistons of brake assemblies to reduce weight, but these pistons suffer from corrosion and heat transfer problems.
The present invention has been devised with the general object of overcoming the above and other disadvantages by the use of nonmetallic pistons which have a low degree of thermal conductivity, are not generally subject to corrosion and can have the ability to absorb moisture from the brake fluid to reduce the possibility of vapour lock.